The invention is broadly applicable to joining glass, metal and ceramic components. However, it is particularly applicable to producing envelopes for cathode ray tubes, and the description is so directed.
It is customary in producing cathode ray tube envelopes to press funnel and faceplate components separately. These components are then joined with a fusion seal employing a mid-temperature sealing glass frit.
Lead-zinc-borate sealing glasses, both crystallizing and non-crystallizing, have been used commercially for this purpose over a long period of time. These glasses have proven very successful for the purpose. However, there has been a continuing search for a sealing material that would retain all of the attributes of the lead glasses, but would further improve on some of these features.
A driving force in this search has been a desire for a glass having an even lower sealing temperature than the lead-zinc-borate type glass. Such a lower temperature glass would be more compatible with thermally sensitive components and coatings present in electronic products such as cathode ray tubes. More recently, the search has been accelerated by the desire to eliminate lead for health and safety reasons.
The materials search led to development of tin-zinc-phosphate glasses as described in U.S. Pat. No. 5,246,890 (Aitken et al.) and No. 5,281,560 (Francis et al.). The glasses described in these patents are lead-free, and provide somewhat lower sealing temperatures in the range of 400.degree.-450.degree. C.
The Aitken et al. glasses are of particular interest for use in producing seals in cathode ray tube envelopes because of their relatively low tin oxide contents. In addition to being lead-free, these glasses have compositions containing 25-50 mole % P.sub.2 O.sub.5 and SnO and ZnO in amounts such that the mole ratio of SnO:ZnO is in the range of 1:1 to 5:1. The glass compositions may further contain up to 20 mole % of modifying oxides including up to 5 mole % SiO.sub.2, up to 20 mole % B.sub.2 O.sub.3, and up to 5 mole % Al.sub.2 O.sub.3. They may also contain one or more crystallization promoters selected from 1 to 5 mole % zircon and/or zirconia and 1-15 mole % R.sub.2 O. Additionally, the composition may include a seal adherence promoter selected from up to 5 mole % WO.sub.3, up to 5 mole % MoO.sub.3, up to 0.10 mole % Ag metal and mixtures.
In producing a sealing material, mill additions to the sealing glass may be made in amounts up to about 30% by weight with no more than 15% being preferred. These additions are made to provide a sealing material having a lower effective coefficient of thermal expansion (CTE). The mill additions include metal pyrophosphate crystalline materials, cordierite, solid solutions of beta-spodumene or beta-eucryptite, silica and quartz glasses and Invar.
The manufacturing process for cathode ray tubes imposes severe viscosity/temperature restraints on a frit intended for use in sealing envelope components. One such restraint arises from the need to conduct the sealing operation at temperatures that do not exceed the strain point of the funnel glass. This requirement, in turn, necessitates that the viscosity of a sealing frit must be in the range of 10.sup.3 -10.sup.4 poises (10.sup.2 -10.sup.3 Pa.s) at a temperature on the order of 450.degree. C. Otherwise, the frit will have insufficient flow to form a seal with a strong hermetic bond.
Following the sealing operation, the panel-funnel assembly is reheated under vacuum to a temperature in the range of 300.degree.-400.degree. C. in an exhaust bake-out process. This bake-out removes volatile constituents of the electronic system, and establishes the needed vacuum level in the tube to assure long tube life. The frit requirement for this second step in the process is essentially the opposite of that needed for successful sealing. To survive the exhaust bake-out, the frit must be rigid at exhaust temperatures. This requires a minimum viscosity of 10.sup.9 poises (10.sup.8 Pa.s), preferably 10.sup.13 poises (10.sup.12 Pa.s), to avoid movement in the seal and resulting breakage or loss of vacuum.
These dual viscosity/temperature requirements are met currently by employing high lead frits in the PbO--ZnO--B.sub.2 O.sub.3 system that form a crystallized seal. These lead frits are initially vitreous, but have a small amount of zircon or alumina added as a mill addition to induce crystallization. The frits exhibit excellent flow during the initial portion of the hold at the 440.degree.-450.degree. sealing temperature. Near the end of this hold period, they undergo rapid crystallization to a degree greater than 95%. This forms a strong, rigid seal which remains rigid during the exhaust bake-out process.
Frits in the SnO--ZnO--P.sub.2 O.sub.5 ternary system exhibit good flow properties at temperatures as low as 360.degree. C. They also have expansion coefficients close to the requisite 95-100.times.10.sup.-7 /.degree. C. range needed for compatibility with current panel and funnel glasses. However, the glass frits are relatively resistant to crystallization and tend to behave essentially as a vitreous flit. As a result, these flits form good seals, but have exhibited a poor survival rate in the exhaust process.
The present invention solves this problem by providing a modified sealing material based on a SnO--ZnO--P.sub.2 O.sub.5 glass frit. The sealing material forms a seal, but then undergoes a substantial change in viscosity/temperature characteristics under thermal influence. The nature of this change is such that the seal remains rigid and does not soften during a subsequent vacuum bake-out process.